ABSTRACT Loeb and Beutner (1911) showed that when the cutinised epidermis of the apple separates two unlike solutions of electrolytes, a potential difference develops, which they interpret as being due to exchange of ions, mainly cations, across the epidermis. They suppose that the faster-moving ions tend to penetrate more rapidly through the epidermis, and hence build up a potential difference. Michaelis (1925) gives a mathematical development of essentially this theory to explain his own experiments with “dried” celloidin membranes which are permeable to cations but not to anions, and which, like the apple epidermis, develop a potential difference which is constant for a considerable period of time. Recently, Mond and Hoffmann (1928) described a membrane made of celloidin and Rhodamin B, which is selectively anion permeable. This was followed by Höber and Hoffmann’s (1928) study of a mosaic membrane made up of alternating areas of cation permeable celloidin and anion permeable celloidin-Rhodamin B. In this case the potential difference obtained was considerably lower than that for either the celloidin or celloidin-Rhodamin B membranes separately, the potential differences of the two areas being in opposite directions and tending to annul one another. The cutinised epidermis from the inner surface of bulb scales of the onion resembles these membranes in that there is no diffusion of ions through the epidermis from solutions of electrolytes into distilled water (Brooks, 1917). It, therefore, seemed desirable to study the nature of the potential differences developed by the onion epidermis when it separates two solutions containing ions, with a view to further analysis of the electromotive mechanisms at work.